So I have noticed even some 05-06's with a failing or dead center high-mounted stop light (the LEDs that are on the parcel shelf in the back windshield). My 03 sedan did the same thing, and for a while I found that hitting the back windshield where the light was made it come back on. When my Bose sub crapped out and I replaced it, I disconnected the CHMSL and when I connected it back VOILA! it was fixed! So before you replace the circuit board at the stealership (Sewell has it listed at $146.21) try this.

Remove the plastic honeycomb cover thing where the sub is (I haven't seen what this looks like in a non-Bose car) there are some clips and adhesive so be careful not to break it like i did!

Unplug the wiring harness that goes to the lights and re-secure it firmly

Replace the honeycomb thing and you're done!

NOTE: this does not pertain to the LEDs in the optional spoiler.

 

Thanks for the potential DIY. Glad it worked for you.

 

those speaker covers are a biatch. remove rear shelf if you dont want to break anything.

rear seat bottoms, backs, C pillar trims. pull up on the shelf to release the pressure clips, then out. it will get snagged on the seat belts anchors. you dont have to remove those.

haven't tried tapping on the rear glass, i've just assumed replace the 3rd lamp assembly.